Add a pull request template

Based on one from ArasP:
http://aras-p.info/blog/2017/05/07/Users-POV-and-Empathy/

.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,16 @@
+### Purpose of this PR
+
+* Description of feature/change.
+* Which part of OpenTK does this affect (Math, OpenGL, Platform, Input, etc).
+* Links to screenshots, design docs, user docs, etc.
+
+### Testing status
+
+* Explanation of what’s tested, how tested and existing or new automation tests.
+* Can include manual testing by self.
+* Specify test plans.
+* Rarely acceptable to have no testing.
+
+### Comments
+
+* Any other comments to help understand the change.